1. Field of the Invention
This invention relates to zoom lenses and, more particularly, to compact large aperture zoom lenses of short total length suited to cameras for photography, video cameras, etc. Still more particularly, it relates to improvement of the optical performance over the entire zooming range at as high an aperture ratio as 1.4.
2. Description of the Related Art
In the field of cameras for photography and video cameras there has been a demand for zoom lenses of large aperture ratio and high range with a high grade of optical performance.
Of these, the video camera, because of its image pickup element being relatively low in sensitivity, has to have a zoom lens of as high an aperture ratio as possible.
Up to now, the 2/3 in. image pickup tube has widely been used in the video cameras from the two standpoints of compactness and image quality. Also, in view of good manageability and a possibility of further reducing the bulk and size, 8 mm video cameras are gradually becoming popular. For adaptation to this type of video cameras, the size of the image pickup tube must be further reduced in such a manner as to maintain good image quality. From this reason, 1/2 in. tube, or 1/2 in. image pickup device has recently found its use in video cameras.
There have been previous proposals for a zoom lens having an F-number of 1.2-1.4 or thereabout and a zoom ratio of about 6 as, for example, in Japanese Laid-Open Patent Applications Nos. Sho 60-51813, corresponding to U.S. Pat. No. 4,621,905, and Sho 60-260912. What is proposed in that patent is a so-called five-unit zoom lens comprising, from front to rear, a first lens unit of positive power for focusing, a second lens unit of negative power for varying the image magnification, a third lens unit for compensating for the image shift resulting from the variation of the image magnification, a fourth lens unit for making afocal the light beam from the third lens unit, and a fifth lens unit for image formation.
The shortening of the total length of the entire lens system have generally been achieved when the size of the first lens unit is minimized. To this end, the F-number may be increased. But to increase the F-number is not very desirable in the case of video cameras which require that the lens system is of large aperture ratio. If the requirements of decreasing the F-number and of minimizing the bulk and size of the entire system are desired to fulfill at once, it becomes important in any case that the various optical constants of each of the lens units constituting the lens system are taken to appropriate values.
Yet, if the fulfillment of the above-mentioned requirements is relied on the mere method of strengthening the refractive power of each individual lens unit, the spherical aberration in the paraxial region of the image frame, and aberrations in the intermediate to the marginal regions such as coma and higher order aberrations such as sagittal halo are increased largely. Hence it becomes very difficult to obtain a high grade optical performance.
For example, as the refractive power of the front or first lens unit is strengthened and, therefore, the overall image magnification of the second lens unit or variator to the last one for image formation has to be increased, the first lens unit produces many large aberrations which cannot be corrected by the following lens units. Further, the tolerances of manufacturing become severer. Hence, it becomes difficult to preserve the optical performance at a predetermined standard.
Particularly in application to the 1/2 in. image pickup element, for the effective diameter .phi..sub.A of the image circle, the total length L from the front vertex to the image plane takes a relatively large value of about L=12.5-14.phi..sub.A. Thus, it becomes difficult to shorten the total length L in such a manner that good optical performance is maintained. As another related technique, there is U.S. patent application Ser. No. 156,935.